1. Field of the Invention
This invention relates to a wellbore positioning system for locating a downhole tool in a borehole.
2. Description of the Related Art
A common problem faced when drilling oil wells is how to locate a downhole tool such as a whipstock inside a casing section repeatedly at the same depth and at the same orientation, for example in order to drill a deviated side track through a window in the casing. Several solutions to this problem have been found. The most basic is a muleshoe and key (see FIG. 1) where the key is set inside the casing in a fixed point and a muleshoe profile is machined into the tool being run, or into a separate muleshoe attachment fitted to the string. The muleshoe profile can alternatively be machined into the inner face of the casing (e.g. upside down with the apex pointing upwards), and a spring loaded key can be provided on the locating tool. In each case, the key comes to rest at a key seat machined into the wall of the muleshoe.
While the muleshoe allows rotational orientation and depth control of the whipstock or other downhole tool on the string, it does not allow multiple vertical locations to be located automatically. In addition, the key can become snagged on the apex of the muleshoe rather than on the key seat. Also, if the muleshoe is machined into the inner face of the casing, which is preferred, there is a loss of effective diameter, and a consequent weakening in the casing. Furthermore, the key seat can become filled with accumulated debris which can prevent proper seating of the key.
In an effort to address the problem of identifying multiple vertical locations, location systems such as the selective key setting tool have been devised. In this tool (shown in FIG. 2), a number of locating profiles are machined into the casing, each having a unique shape, and a locating tool with laterally extending spring-loaded keys is run through the casing until the keys match with the desired locating profile, at which point the keys engage the profile and further downward movement is resisted. While this allows the identification of a particular vertical location corresponding to the key, it does not orient the locating tool or whipstock rotationally.
According to the present invention there is provided a wellbore system for selectively locating a downhole tool in a wellbore casing or tubing comprising a plurality of positioning guides spaced in the wellbore casing or tubing at predetermined depths wherein each positioning guide has a unique profile for selectively receiving a matching profile key of a downhole tool, the guides for each key having a configuration which controls the depth and rotational position of its matching key. The control of the vertical and rotational position of the key thereby controls the vertical and rotational position of the member. The member can be a downhole assembly such as a running tool or a whipstock etc.
The invention also provides a method of selectively locating a downhole tool in a wellbore casing or tubing comprising the steps of:
a) placing a plurality of positioning guides at desired depths in the wellbore casing or tubing wherein each guide has a unique profile for selectively receiving a matching profile locating key,
b) selecting the profile of a locating key for the downhole tool to match the profile of a desired positioning guide, and
c) running the downhole tool to the desired positioning guide wherein the locating key engages the matching profile of the positioning guide which controls the depth and rotational position of the locating key to orient the downhole tool in a desired direction.
Preferably each key guide is a slot or a ledge, which in particular embodiments extend downwards and laterally. In a preferred embodiment the key guides are spiral slots, optionally covering a minimum of 450xc2x0 of circumference.
By casing, we mean any downhole tubular such as casing, liner, drill pipe, or any functional equivalents.
It is an important aspect of the invention that each key guide only allows engagement of a particular key, such that the profile of the key only fits within the profile or width of the key guide such as the slot. In particular cases, there can be more than two key guides, and/or more than two keys, so that multiple outlets spaced vertically and rotationally from one another in the casing string can be identified and particular tools in the work string can be located at different rotational positions and at different depths throughout the string. In one embodiment, the keys can be spring loaded and/or retractable.